1. Field of the Invention
This invention relates to medical devices and more particularly an implantable cardiac pacemaker capable of being programmed in a variety of different states.
2. Description of the Prior Art
The art of implantable cardiac pacemakers was first disclosed by Greatbatch in U.S. Pat. No. 3,057,356 entitled "Medical Cardiac Pacemaker," which issued in 1962. The device disclosed by Greatbatch included a relatively simple relaxation oscillator that generated electrical pulses at a fixed rate. These pulses were applied to the heart through a lead consisting of a conductor wire and an electrode to cause the heart to contract each time a pulse occurred. Since 1962, many improvements to cardiac pacemakers have occurred. These improvements include increased sophistication to the circuitry, including the inclusion of a sense amplifier to interact with the oscillator in providing stimulating pulses only when needed (the demand pacemaker), features to improve the reliability and longevity of pacemakers, improved packaging techniques, better power sources and improved leads and conductors.
Another improvement which has occurred since Greatbatch first disclosed the implantable cardiac pacemaker is means to allow the pacemaker to be reprogrammed after it has been implanted. In U.S. Pat. No. 3,805,796 in the name of Reese Terry, Jr. et al, entitled "Implantable Cardiac Pacer Having Adjustable Operating Parameters," which issued in 1974, circuitry is disclosed to allow the rate of the pacemaker to be noninvasively changed after it has been implanted. The rate varies in response to the number of times a magnetically operable reed switch is closed. The Terry et al device operates by counting the number of times the reed switch is closed and storing that count in a binary counter. Each stage of the counter is connected to either engage or bypass one resistor in a serially connected resistor chain, which chain is a part of the RC time constant controlling the pacemaker rate.
The concept of the Terry et al device has been improved upon by the apparatus shown in U.S. Pat. No. 4,066,086 in the name of John M. Adams et al, entitled "Programmable Body Stimulator," which issued in 1978, and which discloses a programmable cardiac pacemaker that responds to the application of radio frequency (RF) pulse bursts while a magnetic field held in close proximity to a magnetically operated reed switch included within the pacemaker package holds the reed switch closed. In the Adams et al circuit, again only the rate is programmable in response to the number of RF pulse bursts applied. The use of radio frequency signals to program cardiac pacemaker was first disclosed by Wingrove in the U.S. Pat. No. 3,833,005 entitled "Compared Count Digitally Controlled Pacemaker" which issued in 1974. The Wingrove device was capable of having both the rate and pulse width programmed. However, no pacemaker has ever been described which is capable of having more than two parameters programmed or selected features or tests programmed on command. Such a pacemaker could be called a universally programmable pacemaker.
One area where cardiac pacing technology has lagged behind conventional state of electronic technology involves utilization of digital electrical circuits. One reason for this has been the high energy required to operate digital electronic circuits. However, with more recent technology advances in complimentary metal oxide semiconductor (CMOS) devices fabricated on large scale integrated circuits, together with the improvements of cardiac pacemaker batteries, digital electronic circuits are beginning to be utilized in commercial pacemakers. The inherent advantages of digital circuits are their accuracy, and reliability. Typically, the digital circuit is operated in response to a crystal oscillator which provides a very stable frequency over extended periods of time. There have been suggestions in the prior art for utilizing digital techniques in cardiac stimulators and pacemakers since at least 1966. For instance, see the article by Leo F. Walsh and E. Neil Moore, entitled "Digital Timing Unit for Programming Biological Stimulators" in The American Journal of Medical Electronics, First Quarter, 1966, Pages 29 through 34. The first patent suggesting digital techniques is U.S. No. 3,557,796 in the name of John W. Keller, Jr., et al, and is entitled "Digital Counter Driven Pacer," which issued in 1971. This patent discloses an oscillator driving a binary counter. When the counter reaches a certain count, a signal is provided which causes a cardiac stimulator pulse to be provided. At the same time the counter is reset and again begins counting the oscillator pulses. Additionally, in the Keller et al patent, there is disclosed the digital demand concept, in which the counter is reset upon the sensing of a natural heartbeat, and the digital refractory concept, in which the output is inhibited for any certain time after the provision of a cardiac stimulating pulse or the sensing of a natural beat.
As mentioned above, digital programming techniques are shown in both the Terry et al patent 3,805,796 and the Wingrove U.S. Pat. No. 3,833,005. Wingrove additionally discloses digital control circuitry for controlling the rate of the stimulating pulses by providing a resettable counter to continually count up to a certain value that is compared against a value programmed into a storage register. The Wingrove patent also shows provisions for adjusting the output pulse width by switching the resistance in the RC circuit which controls the pulse width.
Other patents disclosing digital techniques useful in cardiac pacing include U.S. Pat. No. 3,631,860 in the name of Michael Lopin entitled "Variable Rate Pacemaker, Counter-Controlled, Variable Rate Pacer;" U.S. Pat. No. 3,857,399 in the name of Fred Zacouto entitled "Heart Pacer;"U.S. Pat. No. 3,865,119 in the name of Bengt Svensson and Gunnar Wallin entitled "Heartbeat Accentuated with Controlled Pulse Amplitude;" U.S. Pat. No. 3,870,050 in the name of Wilson Greatbatch entitled "Demand Pacer;" U.S. Pat. No. 4,038,991 in the name of Robert A. Walters entitled "Cardiac Pacer with Rate Limiting Means;" U.S. Pat. No. 4,043,347 in the name of Alexis M. Renirie entitled "Multiple-Function Demand Pacer with Low Current Drain;" U.S. Pat. No. 4,049,003 in the name of Robert A. Walters et al entitled "Digital Cardiac Pacer;" U.S. Pat. No. 4,049,004 in the name of Robert A. Walters entitled "Implantable Digital Cardiac Pacer Having Externally Selectable Operating Parameters and One Shot Digital Pulse Generator for Use Therein;" and U.S. Pat. No. 4,074,720 in the name of Franklin I. Malchman et al entitled "Cardiac Pacer with Rate Runaway Protection."
3. Summary of the Invention
In the prior art, it is very common to include a magnetically actuated reed switch with a pacemaker. The reed switch can be closed by the placement of a magnet directly over the pacemaker, and when closed by the magnetic force, sends a signal to disable the effects of the sense amplifier. The purpose for including such a reed switch with a demand pacemaker, is to allow the physician to obtain information on the pacemaker pulse when natural cardiac activity is occurring which otherwise would be inhibiting the operation of the pulse generating circuit. With programmable pacemakers, the reed switch is utilized for an additional purpose, and that is that the reed switch must be closed prior to the provision of the programming signal in order to avoid the possibility of inadvertently programming the pacemaker by extraneous signals, such as muscle noise or electromagnetic interference signals.
In the case where the reed switch is used to enable the pacemaker to respond to the programming signals, it sometimes becomes desirable to override the effects of the reed switch in order to allow the physician to observe how the pacemaker responds in the absence of the closed reed switch. Of course, this could occur by simply removing the magnetic head from the vicinity of the pulse generator. However, this causes extra effort on the physician's part, so he would then have to reposition the magnetic head when he desires to reprogram the pulse generator. Further, the physician would not be able to instantaneously react should he program the device and remove the head and then find that an inappropriate program had been transmitted to the pacemaker. Being able to override the effects of the reed switch becomes more important in situations where the physician may desire to temporarily program the pacemaker and observe the response to the temporarily programmed signal. In the pacemaker pulse generator described hereafter, the removal of the magnet from the vicinity of the pulse generator will terminate the temporary programming function. However, this may be undesirable in certain circumstances inasmuch as the physician desires to observe the effect of the temporary command in a mode with the sense amplifier enabled. Particularly, this is important when programming a function which is only observable by noting the response of the sense amplifier, such as when one desires to program the sensitivity of the amplifier.
In accordance with one preferred aspect of this invention, there is provided a programmable cardiac pacemaker pulse generator which is programmed by applying a programming signal thereto while maintaining switch means in a disable position from its normally enable position. The programming signal includes data manifesting desired operating conditions for the pulse generator to assume. The pulse generator comprises oscillator means for providing cardiac stimulating pulses and sensing means responsive to the occurrence of natural activity for providing a signal to the oscillator means to inhibit the provision of cardiac stimulating pulses. In addition, the pulse generator includes inhibiting means responsive to the switch means being in the disable position for preventing the provision of the sensing means signal to the oscillator means unless a selected signal is applied thereto in response to the programming signal.